In recent years, a variety of projectors have been widely used in various video applications. By the projector, an image signal provided by an image signal source can be enlarged and shown on a projection screen. For reducing power consumption and having longer life, the illumination system of the current projector employs a solid-state light-emitting element (e.g. light emitting diode or laser diode) to replace the conventional high intensity discharge (HID) lamp.
Generally, the illumination system of the projector should emit three primary colors of light, i.e. red light (R), green light (G) and blue light (B). As for the luminous efficiency of the solid-state light-emitting element, the luminous efficiency of the blue solid-state light-emitting element is higher than the luminous efficiency of the red solid-state light-emitting element; and the luminous efficiency of each of the blue solid-state light-emitting element and the red solid-state light-emitting element is much higher than the luminous efficiency of the green solid-state light-emitting element. Since the green solid-state light-emitting element has poor luminous efficiency, the green light is produced by using a blue solid-state light-emitting element and a plate containing phosphor coating to excite blue light as green light. That is, in replace of the green blue solid-state light-emitting element, the uses of the blue solid-state light-emitting element and the plate containing phosphor coating may directly emit the green light. Consequently, the luminous efficiency of the whole illumination system is enhanced.
The conventional illumination system, however, still has some drawbacks. For example, during the process of using the phosphor plate to convert the incident ray, a portion of the incident ray is reflected by the phosphor plate. Under this circumstance, an energy loss problem occurs, and thus the luminous efficiency arising from conversion is largely reduced. For solving the energy loss problem from reflection, a reflective optical element is used in the illumination system for returning the reflected light back to the phosphor plate. Since the reflected light is further excited as green light by the phosphor plate, the energy loss will be reduced.
FIG. 1A schematically illustrates the architecture of a conventional illumination system. FIG. 1B schematically illustrates the configuration of a phosphor plate used in the conventional illumination system as shown in FIG. 1A. The conventional illumination system 1 comprises a blue LED 11, a red LED 12, a phosphor plate 13 and a dichroic minor 14. The dichroic mirror 14 is located between the blue LED 11 and the phosphor plate 13. The blue LED 11 is used for emitting blue light. The blue light is transmitted through the dichroic mirror 14 and directed to an optical path. The red LED 12 is used for emitting red light. The red light is reflected by the dichroic mirror 14 and directed to the optical path. The phosphor plate 13 is located at the optical path. In addition, the phosphor plate 13 comprises a phosphor layer 131, a glass layer 132 and a reflective optical element 133. The phosphor layer 131 is used for exciting the blue light and converting the blue light into green light to be outputted. A portion of the incident ray that is reflected by the phosphor layer 131 is reflected back to the phosphor layer 131 by the reflective optical element 133. Since the reflected light is further excited as green light by the phosphor layer 131, the energy loss will be reduced.
Although the conventional illumination system 1 is effective to solve the energy loss problem, there are still some drawbacks. For example, since the reflective optical element 133 is disposed on the incident surface of the glass layer 132, if the incident ray with a large incident angle (e.g. greater than 42 degree) is excited and reflected by the phosphor layer 131 or reflected within the glass layer 132, the incident ray is readily subject to total internal reflection. Under this circumstance, a standing-wave effect is generated and a light leakage problem occurs. Consequently, the luminous efficiency is largely reduced.
FIG. 1C schematically illustrates the occurrence of a light leakage problem in the situation that a incident ray with a large incident angle is excited and reflected by the phosphor layer of the phosphor plate. As shown in FIG. 1C, after the incident rays L1 and L2 are transmitted through the incident surfaces of the reflective optical element 133 and the glass layer 132 to be introduced into the phosphor plate 13, the incident ray L1 is reflected by the phosphor layer 131, reflected within the glass layer 132 and then reflected by the reflective optical element 133. Since the incident angle of the incident ray L1 is very large and the distance between the reflective optical element 133 and the phosphor layer 131 is far, the optical path length of the incident ray L1 is too long. In addition, since the incident ray L1 is reflected by the phosphor layer 131, reflected within the glass layer 132 and then reflected by the reflective optical element 133, the incident ray L1 fails to be effectively excited by the phosphor layer 131. As the incident ray L1 is alternately reflected by the phosphor layer 131 and the reflective optical element 133, the incident ray L1 escapes from the edge of the glass layer 132, and thus the light leakage problem occurs. On the other hand, a portion of the incident ray L2 is excited as green light by the phosphor layer 131. Since the green light generated by the phosphor layer 131 is outputted in a full-angle scattering manner, a portion of the green light has a large incident angle and is directed to the glass layer 132 and the reflective optical element 133. Since the incident angle of the green light is large, a portion of the green light may escapes from the edge of the glass layer 132 according to the above-mentioned principles. Under this circumstance, the light leakage problem also occurs, and thus the luminous efficiency fails to be enhanced.